Steam turbines often are positioned in a series of varying steam pressures such that a high pressure section, an intermediate pressure section, and a low pressure section may be positioned one after another. Steam generally may be extracted from the steam path of the high pressure section and used downstream as a cooling flow. Because the enthalpy of the steam extracted from the steam path may vary substantially, the exact enthalpy of the extracted steam may be difficult to predict with certainty.
Specifically, an amount of overcooling generally may be necessary to provide, for example, that the wheel space temperatures of the intermediate section are maintained within structural requirements. To ensure such, an amount of overcooling may be needed given the uncertainty of the steam path. The overcooling, however, may cause other structural issues such as shell distortion, vibrations, packing damage, etc. These issues may be due to excessive temperature mismatches between the cooling steam temperature and the wheel space metal temperatures.
There is a leakage flow that extends through the gap between the inner and outer turbine shells. This flow includes the inner end-packing ring flow and the corresponding snout leakage flow. This leakage flow is generally considered a waste of energy in the system. To the extent the leakage flow is used, such leakage is used as a direct cooling flow from a single source, i.e., the temperature of the flow may not be adjusted.
There is a desire, therefore, for improved cooling systems and methods. Preferably such an improved system and method may employ the leakage flow in a productive and efficient manner while improving the efficiency of the overall system.